1. Field of the Invention
This invention relates to vertical-cavity surface-emitting lasers (VCSELs) and, in particular, to current confinement in VCSELs and single transverse mode operation thereof.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, free electron, and semiconductor lasers. All lasers have a laser cavity defined by at least two laser cavity mirrors, and an optical gain medium in the laser cavity. The gain medium amplifies electromagnetic waves (light) in the cavity, i.e. provides optical gain, by the phenomenon known as stimulated emission. In semiconductor lasers, a semiconductor active region serves as the gain medium. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers are used for a variety of industrial and scientific applications and can be built with a variety of structures and semiconductor materials.
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
Laser gain (or optical gain) is a measure of how well a gain medium such as an active region amplifies photons by stimulated emission. The primary function of the active region in a semiconductor laser is to provide sufficient laser gain to permit lasing to occur. The active region may employ various materials and structures to provide a suitable collection of atoms or molecules capable of undergoing stimulated emission at a given lasing wavelength, so as to amplify light at this wavelength. The active region may comprise, for example, a superlattice structure, or a single or multiple quantum well (MQW) structure.
Amplification by stimulated emission in the active region of a semiconductor laser is described as follows. The active region contains some electrons at a higher, excited state or energy level, and some at a lower, resting (ground) state or energy level. The number and percentage of excited electrons can be increased by pumping the active region with a pumping energy, such as an electrical current or optical pump. Excited electrons spontaneously fall to a lower state, xe2x80x9crecombiningxe2x80x9d with a hole. Both radiative and non-radiative recombination events occur in the active region. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states.
Stimulated emission occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon. In particular, stimulated emission occurs when a photon with an energy equal to the difference between an electron""s energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon. The second photon will have the same energy and frequency as the original photon, and will also be in phase with the original photon. Thus, when the photons produced by spontaneous electron transition interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. (Viewed as waves, the atom emits a wave having twice the amplitude as that of the original photon interacting with the atom.)
Amplification by stimulated emission requires more photons to be produced by stimulated emission than to be absorbed by lower-state electrons. This condition, known as population inversion, occurs when there are more excited (upper lasing level) electrons than ground-state (lower lasing level) electrons. If there were more lower state than upper state electrons, then more photons would be absorbed by the lower energy electrons (causing upward excitations) than would be produced by stimulated emission. When there is a population inversion, however, enough electrons are in the excited state so as to prevent absorption by ground-state electrons from sabotaging the amplification process. Thus, when population inversion is achieved, stimulated emission will predominate over stimulated absorption, thus producing amplication of light (optical gain). If there is population inversion, lasing is possible, if other necessary conditions are also present.
Population inversion is achieved by applying a sufficient pumping energy to the active region, to raise enough electrons to the excited state. In this manner, an active region amplifies light by stimulated emission. Various forms of pumping energy may be utilized to excite electrons in the active region and to achieve population inversion and lasing. For example, semiconductor lasers of various types may be electrically pumped (EP), by a DC or alternating current. Optical pumping (OP) or other pumping methods, such as electron beam pumping, may also be used. EP semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. As a result of the potential applied, charge carriers (electrons and holes) are injected from opposite directions into an active region. This gives rise to an increase in spontaneous generation of photons, and also increases the number of excited state electrons so as to achieve population inversion.
In a semiconductor laser, an active region is sandwiched between the cavity mirrors, and pumped with a pumping energy to cause population inversion. Photons are spontaneously emitted in the active region. Some of those photons travel in a direction perpendicular to the reflectors of the laser cavity. As a result of the ensuing reflections, the photons travel through the active region multiple times, being amplified by stimulated emission on each pass through the active region. Thus, photons reflecting in the cavity experience gain when they pass through the active region. However, loss is also experienced in the cavity, for example less than perfect (100%) reflectivity of the cavity mirrors introduces loss by absorption, scattering, or even extraction of the output laser beam, which can be about 1% of the coherent cavity light.
Therefore, for lasing to occur, there must be not only gain (amplification by stimulated emission) in the active region, but a enough gain to overcome all losses in the laser cavity as well as allow an output beam to be extracted, while still allowing laser action to continue. The minimum gain provided the active region that will permit lasing, given the cavity losses, is the threshold lasing gain of the laser medium. The wavelength range over which the gain spectrum of the active region exceeds this threshold gain helps define the transverse extent of the optical cavity. (For EP lasers, the lowest drive current level at which the output of the laser results primarily from stimulated emission rather than spontaneous emission is referred to as the lasing threshold current.) When the active region provides the threshold lasing gain, there will be a sufficient amount of radiative recombinations stimulated by photons, so that the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and lasing. This causes coherent light to build up in the resonant cavity formed by the two mirrors, a portion of which passes through one of the mirrors (the xe2x80x9cexitxe2x80x9d mirror) as the output laser beam.
Because a coherent beam makes multiple passes through the optical cavity, an interference-induced longitudinal mode structure or wave is observed. The wave along the laser cavity is a standing EM wave and the cavity of (effective) length L only resonates when the effective optical path difference between the reflected wavefronts is an integral number of whole wavelengths (the effective cavity length or optical path difference takes phase-shifting effects at the mirrors into account). That is, the effective optical path from one mirror to the other and back must be an integer multiple of the wavelength. The set of possible wavelengths that satisfy the standing wave condition is termed the set of longitudinal modes of the cavity. Although there are an infinite number of such wavelengths, only a finite number of these fall within the wavelength range over which the gain spectrum of the active region exceeds the threshold lasing gain. The laser will lase only at one or more of the possible longitudinal (wavelength) modes which fit into this wavelength range.
Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation output is perpendicular to the wafer surface. The most common type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The xe2x80x9cverticalxe2x80x9d direction in a VCSEL is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with xe2x80x9cupxe2x80x9d being typically defined as the direction of epitaxial growth. In some designs, the output laser beam is emitted out of the top side, in which case the top mirror is the exit mirror. In other designs, the laser beam is emitted from the bottom side, in which case the bottom mirror is the exit mirror. The exit mirror typically has slightly lower reflectivity than the other (xe2x80x9cbacksidexe2x80x9d) mirror.
VCSELs have many attractive features compared to edge-emitting lasers, such as low threshold current, single longitudinal mode, a circular output beam profile, scalability to monolithic laser arrays, and ease of fiber coupling. The shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. Because of their short cavity lengths, VCSELs have inherent single-frequency operation. Additionally, because the output is perpendicular to the wafer surface, it is possible to test fabricated VCSELs on the wafer before extensive packaging is done (wafer scale probing), in contrast to edge-emitting lasers, which must be cut from the wafer to test the laser. Also, because the cavity resonator of the VCSEL is perpendicular to the layers, there is no need for the cleaving operation common to edge-emitting lasers.
The VCSEL structure usually consists of an active (optical gain) region or layer sandwiched between two mirrors, such as distributed Bragg reflector (DBR) mirrors. DBRs are typically formed of multiple pairs of layers referred to as mirror pairs; DBRs are sometimes referred to as mirror stacks. The DBR mirrors of a typical VCSEL can be constructed from dielectric (insulating) or semiconductor layers (or a combination of both, including metal mirror sections). The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction, i.e. the DBR comprises alternating layers of high and low indexes of refraction. For semiconductor DBRs, the layers are typically selected so that they are easily lattice matched to the other portions of the VCSEL, to permit epitaxial fabrication thereof.
Both EP and OP VCSEL designs are possible. The two mirrors may be referred to as a top DBR and a bottom DBR; the top DBR often serves as the exit mirror. Because the optical gain is low in a vertical cavity design compared to an edge-emitting laser (because the photons in the cavity pass through the active region for a smaller percentage of the round-trip optical path), the reflectors require a high reflectivity in order to achieve a sufficient level of feedback for the device to lase.
For semiconductor DBRs, the number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity). The difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectivity to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and DBR thickness. Conversely, in a dielectric DBR, a smaller number of mirror pairs can achieve the same reflectivity as a larger number in a semiconductor DBR. However, it is sometimes necessary or desirable to use semiconductor DBRs, despite their lower reflectivity/greater thickness, to conduct current, for example (e.g., in an EP VCSEL). Semiconductor DBRs also have higher thermal (heat) conductivity than do dielectric DBRs, making them more desirable for heat-removal purposes, other things being equal. Semiconductor DBRs may also be preferred for manufacturing reasons (e.g., a thicker DBR may be needed for support) or fabrication reasons (e.g., an epitaxial, i.e. semiconductor, DBR may be needed if other epitaxial layers need to be grown on top of the DBR).
When properly designed, these mirror pairs will cause a desired reflectivity at the laser wavelength. VCSEL mirrors are typically designed so that the bottom (backside) DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectivity, while the top (exit) DBR mirror has a reflectivity that may be 98%-99.5% (depending on the details of the laser design). The partially reflective top mirror passes a portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors. Of course, as noted above, in other designs, the bottom mirror may serve as the exit mirror, with the top mirror having the higher reflectivity.
In a VCSEL, it is often preferable for lasing to occur in only a single longitudinal and lateral (transverse) lasing mode (i.e., a fundamental mode) at or near desired resonant wavelength; the fundamental transverse electromagnetic mode (TEM) is also known as the TEM00 mode. While the longitudinal modes correspond to standing waves between the laser mirrors, the TEM modes indicate the spatial or transverse distribution of intensity, in cross section of the beam, perpendicular to the optical axis of the laser. The intensity distribution of the fundamental transverse mode TEM00 is Gaussian.
The transverse extent of the optical cavity must be defined in a VCSEL. The transverse lasing mode is often defined by implementing a means of confinement for photons and/or electrons. Electrical or current confinement is also desirable for EP VCSELs, in which electrical current is used to provide the means of pumping the active region to achieve gain. In an EP VCSEL, for example, top and bottom electrical contacts are typically provided above and below the active region so that a pumping current can be applied through the active region. Current-confinement approaches attempt to create a current-confinement structure to confine the pumping current into a relatively small area of the active region. Confining the current in this way is also sometimes referred to as current guiding or funneling. Such a structure may be employed in a VCSEL to block current flow through an annular perimeter region of the VCSEL structure, and to guide or confine the current to a more or less cylindrical more-conductive region in the center, which may be referred to as a current aperture or current-confinement aperture. Current confinement can provide lower threshold and higher efficiency, and can help the VCSEL operate with a single transverse mode.
Various techniques have been used to achieve electrical or current confinement, including ion implantation (irradiation) and oxidation approaches, as described in Vertical-Cavity Surface-Emitting Lasers: Design, Fabrication, Characterization, and Applications, eds. Carl W. Wilmsen, Henryk Temkin and Larry A. Coldren, Cambridge: Cambridge University Press, chapter 5 (1999); Y. H. Lee et al., Electr. Lett., vol. 26, no. 11, pp. 710-711 (1990); and T. E. Sale, Vertical Cavity Surface Emitting Lasers, Research Press Ltd., pp. 117-127 (1995). Jewell et al., U.S. Pat. No. 4,949,350, for example, describes one current-confinement approach in which a post is etched through the structure. In another technique for current confinement, patterned ion implantation into the p-doped semiconductor top DBR mirror is used to change to properties of some of the semiconductor layers to make them more resistive. This forms an apertured, relatively high-resistivity ion-implanted current-confinement region, which guides the current predominantly through this aperture. In such an approach, implantation of ions into the top DBR mirror can render the material around the laser cavity (or an aperture thereof) relatively nonconductive, thus concentrating the injected current into the active medium. For example, in this ion-implanted approach, light ions such as protons are implanted to relatively deep depths (e.g., about 3 xcexcm), into selected areas of the semiconductor laser surface, damaging the semiconductor crystal lattice and thus rendering those areas relatively nonconductive (more resistive or insulative). This forms a layer having a relatively resistive annular portion and a relatively conductive central aperture portion. Such implants thus define the transverse extent of the laser cavity. Other ion-implanted approaches are taught in Chirovsky et al., U.S. Pat. No. 6,169,756 B1 and Y. Qian et al., Appl. Phys. Lett., vol. 71, no. 1, pp. 25-27 (1997).
Various oxide confinement based approaches have also been explored. In some approaches, insulating oxide layers can be located immediately adjacent to the active region, reducing current spreading and thus giving rise to lower threshold currents that are typically required with conventional ion-implanted VCSELs. One current-confinement approach (typically not utilized with VCSELs, however), employs an apertured, high resistivity oxide layer. It is formed by growing an oxide surface layer, then, using mask technology, etching away selected portions of the oxide. The oxide material is insulative (resistive) so current flows only into those areas of the underlying laser structure where the oxide layer has been removed (the aperture). In another approach, the native oxidation of AlGaAs layers may be used to form a resistive layer between the reflector and the active region. See for example D. Huffaker et al., Appl Phys. Lett., vol. 65, no. 1, pp. 97-99 (1994) and K. D. Choquette et al., Electr. Lett., vol. 30 no.24, pp. 2043-2044 (1994). Selective etching and undercutting of specific layers has also been proposed to form the current guiding aperture. See, for example, Scott et al., U.S. Pat. No. 5,594,751 and H. Deng et al., Electr. Lett., vol. 32, no. 10, pp. 900-901 (1996).
As noted above, it is often desirable to achieve single transverse optical or lasing mode. Refractive index guiding may be used to guide the transverse lasing modes (e.g., to confine the optical mode similar to the manner in which an optical fiber guides light). For example, by etching a mesa into the VCSEL, the mesa itself can serve to confine the transverse optical modes, i.e. define the beam waist of the output laser radiation. However, index guiding is not always possible, practical, feasible, desirable, or sufficient.
Additionally, by confining the pumping current with a current-confinement structure, there will be some gain guiding of the transverse modes. However, such gain guiding is not always sufficient to achieve the desired transverse lasing mode. For example, it is generally desired to achieve as high as possible an output power, while maintaining single transverse mode operation. Single mode operation is important, for example, in telecommunications applications in which the laser output is to be coupled to a single-mode optical fiber. A larger current-confinement aperture will generally allow a greater pumping current to be employed and thus a greater output power to be obtained. On the other hand, the wider the current-confinement aperture, the more gain there is of higher-order modes. When the current-confinement aperture is wide enough, there will be multimode lasing. Thus, for a given VCSEL structure, there is a maximum current-confinement aperture size at which single transverse lasing mode operation is possible. The maximum single-mode current-confinement aperture size also sets an upper limit on the VCSEL""s output power.
There is, therefore, a need for an improved VCSEL techniques and structures for maximizing output power and achieving single transverse lasing mode operation.